Flow of Groundwater through North Slope, Alaska Gravel Pads

Since the beginning of oil production in Alaska, oil companies constructed facilities all across the North Slope of Alaska to access the numerous crude reserves on the North Slope. Production in some of the older fields is on the decline; however exploration in newly opened areas of the North Slope is resulting in the discovery of new reserves. The result is that in the coming decades the old sites will need to be decommissioned as production transitions to new sites. New sites will also need to be designed and constructed. To guard against thawing permafrost and associated thaw subsidence, the oil facilities in the Arctic are constructed on gravel pads placed on top of the existing arctic tundra. Gravel pad thicknesses can range from around 0.6 m to as much as 1.5 m. As the oil companies transition to the new sites the question arises, what should be done with vacated gravel pads? By Alaska State constitution, the natural resources of Alaska belong to the state but the state legislature my lease out land and mineral rights to private entities. Oil companies on the North Slope have historically conducted operations through these leases given by the legislature. The leases require that once resource extraction operations are completed, the facilities must be decommissioned and the sites restored. This original lessee retains this responsibility (regardless if the original lessee has sold or transferred the lease to another entity).

The construction of gravel pads essentially destroys underlying arctic tundra. In undisturbed areas in the Arctic, the tundra itself creates an incredible insulating layer that limits the seasonal thaw depth to around 0.5 m. Removal of this layer causes thaw depths to greatly increase impacting the stability of the ground and the hydrology of the surrounding area. Because of this impact, other possible restoration techniques are being considered, such as vegetating and leaving the pads in place. A major aspect of the gravel pad decommissioning and restoration process is understanding the hydrology in areas where gravel pads exist, in particular, groundwater flow through the gravel pads. Because of the unique conditions in the Arctic, groundwater flow through these gravel pads is not well understood. The purpose of this project is to develop this understanding. We use field measurements and an associated groundwater model to examine the flow of groundwater through gravel pads and the driving forces for this flow. The gravel pad used for this study is located in Prudhoe Bay and is part of the pad constructed for the first production well in Prudhoe Bay. The results from this field study and model will assist engineers and environmental scientists in better understanding the groundwater flow to aid in the decommissioning and restoration process and help inform decision-making in regards to the future of the pads.